Multiphase electrolytic removal apparatus



Oct. 26, 1965 L. A. WILLIAMS MULTIPHASE ELECTROLYTIC REMOVAL APPARATUS Filed Sept. 10, 1962 2 Sheets-Sheet 1 INVENTOR:

BY WIIIJ/I Oct. 26, 1965 1.. A. WILLIAMS 3,214,361

MULTIPHASE ELECTROLYTIC REMOVAL APPARATUS l? 1 k 6, 70T: "Jan-m 7 P TANK Z 52 955 551mm -T 9 IIYVEMTOR: Q Z/zflmvw United States Patent 3,214,361 MULTIPHASE ELECTROLYTIC REMOVAL APPARATUS Lynn A. Williams, Winnetka, lll., assignor to Anocut Engineering (Company, Chicago, 11]., a corporation of Illinois Filed Sept. 19, 1962, Ser. No. 222,319 14 Claims. (Cl. 2tl4224) The present invention relates to improvements in the electrolytic removal of material from an electrically conductive and electrochemically erodable workpiece, and in particular, it relates to the utilization of three phase or multiple phase alternating current in such electrolytic material removal.

This invention is an extension of the invention disclosed and claimed in the copending application of Lynn A. Williams, Serial No. 187,902, filed April 16, 1962, entitled Electrolytic Removal Apparatus and Method, wherein the use and control of alternating current in the electrolytic material removal field is disclosed and considered in detail. This application in some respects is to be considered an extension and continuation in part of that application, which shows in FIG. 9 the use of three phase alternating current for the simultaneous shaping of three separate workpieces. This previously filed application also discusses the advantages in the use of alternating current over direct current in the electrolytic process of material removal and the disclosure of that application is hereby incorporated by reference in this application.

This invention also constitutes an improvement over the use of alternating current in the electrolytic process of material removal as disclosed in conjunction with FIG. 23 of the copending application of Lynn A. William, Serial No. 772,960, filed November 10, 1958, entitled Electrolytic Shaping, now United States Patent No. 3,058,895, dated October 16, 1962.

The use of alternating current for the simultaneous electrolytic erosion of separate faces of a single workpiece has proved to be advantageous for the separate faces will be alternately acted upon. When only one face is to be electrolytically machined using singl phase alternating current, the current will be effective only half the time. This difficulty is eliminated by the present invention.

It is also possible, with this invention, to connect the electric power source and the electrolyte feed to the electrode through the ram, thus eliminating any connections in the work area to the workpiece itself.

It is customary to supply factories and manufacturing establishments having heavy power requirements with three phase alternating current. The previous invention, primarily concerned with single phase current, required the production or separation of a single phase from the three phase source. The present invention contemplates the use of all phases of the power supply, thus eliminating the costly rectifier to produce direct current and the arrangements ncessary to provide single phase alternating current.

It is, therefore, a principal object of the present invention to provide a new and improved electrolytic removal or shaping apparatus and method wherein the power supply is a multiple phase alternating current and the workpiece erosion may be accomplished with a single electrode structure.

Another object is to provide a new and improved electrolytic workpiece shaping apparatus incorporating an electrode structure having a plurality of separate work faces or work face areas each connected to receive at any one time a single phase of a multiple phase alternating current power supply.

Another object is to provide a new and improved elec- 3,214,361 Patented Oct. 26, 1965 trolytic workpiece shaping apparatus incorporating a single electrode structure having a plurality of work faces or work face area-s corresponding in number to the product of the numbers of phases in a multiple phase alternating current power supply and a whole number.

Another object is to provide a new and improved electrolytic workpiece shaping apparatus incorporating a composite electrode structure having a working face comprised of a plurality of tip elements each connected to receive only one phase of a multiple phase alternating current and sequentially to be relatively negative and positive with respect to the workpiece and the other tip elements.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings, wherein FIG. 1 is a schematic illustration of a portion of an electrolytic shaping and cavity sinking apparatus incorporating a multiple work face electrode connected to a three phase power 4 supply;

FIG. 2 is a greatly enlarged transversesectional view through the electrode, and may be considered as being taken along the line 2-2 of FIG. 1, looking in the direction of the arrows;

FIG. 2a is a view on a smaller scale, similar to FIG. 2, illustrating a different number of electrode elements;

FIG. 3 is a medial sectional view through the electrode and may be considered as being taken along the line 3-3 of FIG. 2, looking in the direction of the arrows;

FIG. 4 is a detail sectional view taken along the line 4-4 of FIG. 3, looking in the direction of the arrows;

FIG. 5 is a schematic view similar to FIG. 1, showing another form of multi-work face electrode connected to receive three phase alternating current;

FIG. 6 is a transverse sectional view through the electrode of FIG. 5, and may be considered as being taken along the line 66 of FIG. 5, looking in the direction of the arrows; and

FIG. 7 is a longitudinal sectional view on a greatly enlarged scale through the apparatus of FIG. 5, and may be considered as being taken along the line '7-'7 of FIG. 5, looking in the direction of the arrows.

Referring to the drawings, and particularly to FIGS. 1 through 4, it will be seen in FIG. 1 that the electrolytic apparatus 20, indicated schematically, is of the type shown in the copending application of Lynn A. Williams and Leonard Malkowski, Serial No. 201,679, filed June 11, 1962, for an Electrolytic Shaping and Cavity Sinking Apparatus." It includes a ram 22 having a ram head 24 mounted on the forward end thereof and adapted to be moved toward and away from a workpiece W, which is mounted in a work area as more fully explained in that application. A manifold 26 is mounted on the ram head 24 and preferably is made of an electrically insulating material which is also resistant to erosion. It has been found that among the materials which are particularly suitable for this are Teflon and epoxy resins. Mounted on the manifold 26 is an electrode structure 23.

The workpiece W is mounted on a holder or chuck 30, the particular form of which is not important for consideration of the present invention, but which may be of the type disclosed in the copending application of Lynn A. Williams, entitled Workpiece Holding Fixture, Serial No. 228,401, filed October 4, 1962. It is carried at one end of a rotatable shaft 32 which is driven by a motor 34 through a belt 36 and driving and driven pulleys 38 and 40. It is contemplated that for some uses of the present invention, relatively rotating the workpiece W and the electrode structure 28 produces desirable results, while in other installations such rotation is not necessary.

The electrode structure 28 includes a plurality of electrically conductive segments 42 secured to the manifold 26, for example by bolts 44, and insulated from each other by layers 46 of insulation which may be bonded to one or the other, or both, of the segment faces in juxtaposition to one another, or may comprise separate insulating inserts. The number of segments 42 must equal the product of the number of phases in the alternating current supply and a whole number. The customary commercial alternating current supply is three phase, and consequently the number of segments 42 will be divisible by 3.

The segments 42 are preferably made of copper or similar good conducting material, and each is fitted with a working tip 48 of a conductive material which is electrochemically inert, such as graphite. The necessity for the electrically conductive, but electrochemically inert, working tips 48 is understandable when it is appreciated that these tips are alternately negative and positive with respect to the surface of the workpiece being shaped. When the tip is negative with'respect to the workpiece, the area of the latter adjacent the tip is positive and material is removed from the workpiece surface area. However, when the tip is positive and the adjacent workpiece area is negative, relatively speaking there will be no material removed from the workpiece area, and in order to prevent material being removed from the electrode structure, the tip must be electrochemically inactive and inert. It has been found that graphite working tips are usable for this purpose. It is also possible to use graphite plated on its exposed surface with rhodium or platinum. Or instead of graphite, titanium plated with rhodium or platinum may be used, taking care in all cases to employ electrolytes which will not cause attack on the electrodes when they are anodic relative to the work or an adjoining electrode element.

It will be observed from FIGS. 2a and 3 that the electrode structure 28 formed of the plurality of segments 42 and tips 48 is hollow and has a bore 50 through the center thereof. This bore has its surface provided with an electrical insulating coating 52, such as an epoxy resin, and the outer surface of the electrode structure is similarly electrically insulated at 54 in order to prevent stray electrolysis and also the electrolytic erosion of the copper segments. The bore 50 communicates with a chamber 56 in the manitold 26, which in turn is connected by a passage 58, a fitting 60, and a hose 62, to a source of electrolyte under pressure, such as a tank 621 and pump 62p, which are diagrammatically shown, and filters, pressure controlling gauges, valves, and the like, as more fully disclosed in the copending applications of Lynn A. Williams, Serial No. 772,960, filed November 10, 1958, for Electrolytic Shaping, and Serial No. 73,154, filed September 2, 1960, for Electrolytic Cavity Sinking Apparatus and Method.

The three phase source of alternating current, which preferably includes a conventional stepdown transformer (not shown) to reduce the usable voltage to the range of 3 to 25 volts, is connected to a plurality of conductors 64a, 64b, and 640 which in turn are connected to the separate terminals 66a, 66b, etc., the third or terminal not being shown. These terminals connection each conductor directly to a single segment and they should be arranged so that as the electrode is viewed in plan view, the electrode segments will be connected sequentially in the series of a, b, and c, around the electrode, thus insuring that there is a proper distribution of these multiple current phases in the electrode structure and that at no time during the shaping operation would any adjacent pairs of segments 42 and their working tips 48 have a polarity other than that which would be indicated by the sequential distribution of the phases from the power source, as might be the case were adjacent segments to be connected to the conductor 64a or 64b or 64c.

The illustrations in FIGS. 2 and 3 indicated that the electrode structure 28 is of rather substantial size, as indeed it might well be. However, it should be appreciated that the exterior diameter of the entire electrode may be less than one-half inch, and under these circumstances it might prove adequate that the number of segments be three. On

4: the other hand, if the exterior diameter of the electrode is two or three inches, or even greater, then it is clearly indicated that the number of segments should be relatively large in order to prevent the segments 42 and their working tips 43 from being large in transverse area.

When the workpiece is thin, then to avoid resistance losses of current passing through the work from one large electrode segment to another, some distance away, an electrode of the type shown in FIGS. 1 through 4 is used, with a large number of segments or poles, such as 9, 12, 15, or even 24, insulated from each other as indicated at 46, and a typical example of which is shown in FIG. 2a.

This large number of segments are connected into the power circuit as previously described with the connections being a, b, c, a, b, c, etc., and during the electrolytic action on the workpiece the lateral distance that the current has to travel through the workpiece itself is relatively short. For example, if the segment connected into the circuit a is positive, while that connected into the circuits b and c is negative, the current will travel from a through the workpiece to b and c, thus removing the material opposite the working faces 48 connected to the b and c terminals and conductors. The same is true if a and b are positive while 0 is negative. Under these circumstances, current is connected from a and b through the workpiece to the 0 connected working tip, at which time the material is removed from the workpiece opposite the c connected working tip.

It will thus be seen that the material removal is not equal in amount at any one time from any particular workpiece area. However, over the entire operation the removal is equalized so that the amount of material removed will be the same over the entire surface of the workpiece being eroded.

It will be appreciated that when shaping a surface such as that just described, it will be advisable, if not desirable, that the workpiece be rotated, thus removing any marks which might be left by the small spaces between the segments 42 and their respective working tips 48.

FIGS. 5 through 7 illustrate another form of apparatus indicated at 68, which includes a ram 22 having a head 24 to which is mounted an electrode structure 70, which is longitudinal or rectangular in its configuration.

The workpiece W is held against a table 72 by clamping devices 74 which act against a guide bushing or dam 76 for directing the supply of electrolyte to the work gap between the working face of the electrode structure and the workpiece W. The bushing or dam 76 is made of insulating material such as Teflon, an epoxy resin, or the like. The workpiece is insulated from the table 72 by a layer '78 of insulation, such as glass fiber.

The electrode structure 70 comprises a plurality of segments 80 which may be copper bars secured to an insulating block 82 of Teflon or epoxy resin by bolts 84. The mounting block 82 is in turn bolted to the ram head 24. At their opposite ends the bars or segments 80 are fitted with working tips 84 of graphite or some other electrically conductive and electrochemically inert material.

The electrode segments 80 are insulated from each other at 86, and are provided with an external coating 88 of insulating material such as epoxy resin. The segments are secured together by appropriate clamps 90, and it may be appreciated, therefore, that this electrode 70 may take a variety of shapes, forms, and sizes by selecting the appropriate number of segments 8t? which preferably are insulated on all four sides, and securing them together by the clamps 90. It is, of course, desirable that the segments 80 be relatively small in the transverse direction so that the distance the electric current fiows from positive to negative terminals in the workpiece is short for the reasons previously discussed herein. It will also be noted that the tips 84 project laterally from the electrode structure at its exterior to provide a side cutting flange which insures adequate clearance for the electrolyte flow in the cavity formed in the workpiece.

The threephase alternating current source, which preferably includes a stepdown transformer (not shown), is connected by conductors 92a, 92b, and 92c, to the individual segments at the terminals 94a, 94b, and 940. This electrode is connected and functions in precisely the same manner as that described with respect to FIGS. 1 through 4, and the description of the function therefore will not be repeated.

Since the electrode is solid when composed of the clamped together segments 80, the dam or die bushing 76 is provided for introducing and directing the electrolyte into the work gap between the working tips 84 of the electrode and the workpiece W. The dam 76 has an opening 96 through which the electrode '70 extends or projects, and this opening has a sufficiently snug fit with the electrode so as to prevent the uncontrolled escape of electrolyte from cavity 98 in the dam which accommodates the working tips 84 which are slightly larger than the segments 80 at the periphery of the composite electrode structure, and also to insure that there is a proper supply of electrolyte under pressure at all times. The electrolyte is introduced through passageways 109, leading to the cavity 98, fittings 102, and hoses 104 con nected to the source of electrolyte, including a tank 104: and a pump 104p, which are diagrammatically shown, and filters, valves, gauges, and the like, as described in the aforementioned copending applications of Lynn A. Williams. It Will be noted in this illustration that there are several hose connections to the dam, thereby to insure an adequate supply of electrolyte across the entire surface of the workpiece in which a cavity is being formed.

The electrolyte is permittted to escape from the work gap and also from the cavity 98 through outlet passages 106, fittings 108, and hoses 110. If desired, some restriction may be placed in the outlet passage Title, or hoses 110such as a flow restricting valve-in order to place a back pressure upon the electrolyte in the work gap. Such back pressure has been found to enhance the smoothness of the surface being eroded, thereby giving it a more lustrous appearance, and in some instances, a polished surface such as might be obtained when cavities and other shapes are formed in a material such as 188 stainless steel, for example.

While each of the embodiments described calls for a unitary electrode structure, it is also within the scope of the invention to utilize three (or a multiple of three) electrode elements which are not mechanically fitted together into a unitary structure, but Which are separated one from another to make separated cavities in the same workpiece or in multiple workpieces which are electrically connected together. If the electrodes are tobe fed at uniform rates, then their frontal areas should be equal.

It will be seen from the foregoing description that the advantages and objectives which were claimed for this invention at the outset of this sepcification have been obtained.

While preferred embodiments of the electrolytic removal apparatus constituting this invention have been shown and described, it will be apparent that numerous variations and modifications may be made therein without departing from the underlying principles of the invention. It is therefore desired, by the following claims, to include within the scope of the invention all such modifications and variations by which substantially the results of this invention may be obtained through the use of substantially the same or equivalent means.

What is new and desired to be claimed by United States Letters Patent is:

1. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an electrolyte under pressure, the combination comprising, means for supporting the workpiece, an electrode structure having at least three electrically conductive and electrochemically inert working tips each provided with a working face, means insulating said tips from each other, means for supporting said electrode structure so that said Working faces may be brought into close spacing relationship with the workpiece, electric circuit means adapted to connect a source of three phase alternating current with said electrode working tips in such manner that only one phase is placed on any one tip at any one time and said tpis are rendered alternately negative and positive with respect to the Workpiece, the number of working tips being equal to the product of a whole number and three, and said working tips being so arranged in said electrode structure that of adjacent tips no more than two may have the same polarity at any one time.

2. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an electrolyte under pressure, the combination comprising, means for supporting the workpiece, an electrode structure having at least three electrically conductive segments each having secured thereto an electrically conductive and electrochemically inert working tip defining a working face, means insulating said electrode segments and tips from each other, means for supporting said electrode segments so that said tip faces may be brought into close spacing relationship with the workpiece, electric circuit means adapted to connect a source of three phase alternating current with said electrode segments and said working tips in such manner that only one phase is placed on any one tip at any one time and said tips are rendered alternately negative and positive with respect to the workpiece, the number of segments and working tips being equal to the product of a whole number and three, and said segments and said working tips being so arranged in said electrode structure that of adjacent tips no more than two may have the same polarity at any one time.

3. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an electrolyte under pressure, the combination comprising, means for supporting the workpiece, an electrode structure having at least three electrically conductive and electrochemically inert working tips each provided with a working face, means insulating said working tips from each other, means for supporting said electrode structure so that said working faces may be brought into close spacing relationship with the workpiece to define a narrow work gap between said working faces and the workpiece, means connected to the work gap for pumping an electrolyte therethrough, electric circuit means adapted to connect a source of three phase alternating current with said electrode working tips in such manner that only one phase is placed on any one tip at any one time and said tips are renedered alternately negative and positive with respect to the workpiece, the number of working tips being equal to the product of a whole number and three, and said working tips being so arranged in said electrode structure that of adjacent tips no more than two may have the same polarity at any one time.

4. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an electrolyte under pressure, the combination comprising, means for rotatably supporting the workpiece, an annular electrode structure having a central bore and at least three electrically conductive and electrochemically inert working tips each provided with a working face, means insulating said tips from each other, means for supporting said electrode structure so that said working faces may be brought into close spacing relationship with the workpiece to define a narrow work gap between said working faces and the workpiece, means connected to said electrode structure for pumping an electrolyte through said bore and through the work gap, electric circuit means adapted to connect a source of three phase alternating current with said electrode working tips in such manner that only one phase is placed on any one tip at any one time, the number of working tips being equal to the product of a whole number and three, and said working tips being so arranged about said annular electrode structure that during the electrolytic action said tips are alternately and sequentially negative and positive With respect to adjacent areas of the workpiece.

5. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an eletrolyte under pressure, the combination comprising, means for rotatably supporting the workpiece, an annular electrode structure having at least three electrically conductive segments arranged to define an annulus having a bore therethrough, each segment having an electrically conductive and electrochemically inert working tip defining a working face, means insulating said electrode elements and tips from each other, a manifold connected to and supporting said electrode segments so that said working faces may be brought into close spacing relationship with the workpiece to define a narrow work gap between said working faces and the workpiece, the interior of said manifold being in communication with said electrode bore, means connected to said manifold for pumping an electrolyte under pressure through said bore and through the work gap, electric circuit means adapted to connect a source of three phase alternating current with said electrode segments and said working tips in such manner that only one phase is placed on any one tip at any one time, and number of segments and working tips being equal to the produce of a whole number and three, and said working tips being so arranged about said annular electrode structure that during the electrolytic action said tips are alternatively and sequentially negative and positive With respect to adjacent areas of the workpiece.

6. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an electrolyte under pressure, the combination comprising, means for supporting the workpiece, a complete electrode structure having at least three electrically conductive segments each having secured thereto an electrically conductive and electrochemically inert working tip defining a working face, means clamping said segments into said structure, means insulating said segments and tips from each other, means for supporting said electrode structure so that said working faces may be brought into close spacing relationship with the workpiece, electric circuit means adapted to connect a source of three phase alternating current with said electrode segments and said working tips in such manner that only one phase is placed on any one tip at any one time and said tips are rendered alternately negative and positive with respect to the workpiece, the number of working tips being equal to the product of a whole number and three, and said segments and said working tips being so arranged in said electrode structure that of adjacent tips no more than two may have the same polarity at any one time.

7. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of multiple phase alternating current and an electrolyte under pressure, the combination comprising, means for supporting the workpiece, a composite electrode structure having at least three electrically conductive segments each having secured thereto an electrically conductive and electrochemically inert working tip defining a working face, means clamping said segments into said electrode structure, means insulating said segments and tips from each other, means for supporting said electrode structure so that said working faces may be brought into close spacing relationship with the workpiece to define a narrow work gap between said working faces and the workpiece, guide and dam means for guiding said electrode structure and for directing electrolyte to and through the work gap, means connected to said guide and dam means for pumping an electrolyte thereto, electric circuit means adapted to connect a source of three phase alternating current with said electrode segments and said working tips in such manner that only one phase is placed on any one tip at any one time and said tips are rendered alternately negative and positive with respect to the workpiece, the number of working tips being equal to the product of a whole number and three, and said working tips being so arranged in said electrode structure that of adjacent tips no more than two may have the same polarity at any one time.

8. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of three phase alternating current and an electrolyte maintained between the workpiece and an electrode, the combination comprising, means for supporting the workpiece, an electrode array comprising at least three electrode elements or a whole number multiple thereof, means insulating said elements from each other, means mounting said electrode elements and adapted to move said electrode elements toward the workpiece, each said electrode element having that portion of its surface which is exposed to electrolyte made of a material which is not anodically attacked and the working surface thereof being electrically conductive, and electrical connections to each of said electrode elements and adapted to connect one phase of the three phase alternating current source to each element or to multiples thereof so that said elements are rendered alternately negative and positive with respect to the workpiece, whereby when multiphase alternating current is supplied the current will pass from one or more of said electrode elements through the electrolyte, then through the workpiece, and then back through the electrolyte to one or more other electrode elements.

9. The combination claimed in claim 8, wherein said electrode element surfaces exposed to electrolyte are made of graphite.

Ill. The combination claimed in claim 8, wherein said electrode element surfaces exposed to electrolyte are made of titanium plated to each said element with rhodium or platinum.

111. In an apparatus for electrolytically removing material from an electrically conductive and electrochemically erodable workpiece by the use of three phase alternating current and an electrolyte maintained between the workpiece and an electrode, the combination comprising, means for supporting the workpiece, an electrode assembly comprising at least three electrode elements or a whole number multiple thereof, means insulating said electrode elements from each other, means separately mounting said electrode elements and adapted relatively to move said elements and the workpiece toward each other, each said electrode element having that portion of its surface which is exposed to electrolyte made of a material which is electrically conductive and electrochemically inert in the presence of the electrolyte, and electrical connections to each of said electrode elements and adapted to connect one phase of the three phase alternating current source to each element or to said whole number multiple thereof so that said elements are rendered alternately negative and positive with respect to the workpiece, whereby when three phase alternating current is supplied the current will pass from one or more of said electrode elements through the electrolyte, then through the workpiece, and then back through the electrolyte to one or more other electrode elements.

12. The combination claimed in claim 11, wherein said electrode elements comprise electrically conductive bars, each secured at one end to said mounting means, the other ends of said bars providing said surfaces exposed to electrolyte and being of equal area.

13. The combination claimed in claim 12, wherein said surfaces are made of graphite.

14. The combination claimed in claim 12, wherein said surfaces are made of titanium plated to the body of each bar with rhodium or platinum.

References Cited by the Examiner UNITED STATES PATENTS 3,058,895 10/62 Williams 204-224 X JOHN H. MACK, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,214,361 October 26, 1965 Lynn Ac Williams It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 33, for 'WUGS, 2a and 3" read FIGS 2, 2a, and 3 line 5 for "connection" read connect column 6, line 10, for "tpis" read tips line 57, for

"renedered" read rendered column 7, line 17, for

"eletrolyte" read electrolyte line 36, for "and" read the line 37, for "produce" read product line 48, for "Complete" read composite Signed and sealed this 2nd day of August 19660 QHQUQ Anna:

ERNESF[SHDER EDUUU)J.BRENNER Attesting Officer Commissioner of Patents 

1. IN AN APPARATUS FOR ELECTROLYTICALLY REMOVING MATERIAL FROM AN ELECTRICALLY CONDUCTIVE AND ELECTROCHEMICALLY ERODABLE WORKPIECE BY THE USE OF MULTIPLE PHASE ALTERNATING CURRENT AND AN ELECTROLYTE UNDER PRESSURE, THE COMBINATION COMPRISING, MEANS FOR SUPPORTING THE WORKPIECE, AND ELECTRODE STRUCTURE HAVING AT LEAST THREE ELECTRICALLY CONDUCTIVE AND ELECTROCHEMICALLY INERT WORKING TIPS EACH PROVIDED WITH A WORKING FACE, MEANS INSULATING SAID TIPS FROM EACH OTHER, MEANS FOR SUPPORTING SAID ELECTRODE STRUCTURE SO THAT SAID WORKING FACES MAY BE BROUGHT INTO THE CLOSE SPACING RELATIONSHIP WITH THE WORKPIECE, ELECTRIC CIRCUIT MEANS ADAPTED TO CONNECT A SOURCE OF THREE PHASE ALTERNATING CURRENT WITH SAID ELECTRODE WORKING TIPS IN SUCH MANNER THAT ONLY ONE PHASE IS PLACED ON ANY ONE TIP AT ANY ONE TIME AND SAID TPIS ARE RENDERED ALTERNATELY NEGATIVE AND POSITIVE WITH RESPECT TO THE WORKPIECE, THE NUMBER OF WORKING TIPS BEING EQUAL TO THE PRODUCT OF A WHOLE NUMBER AND THREE, AND SAID WORKING TIPS BEING SO ARRANGED IN SAID ELECTRODE STRUCTURE THAT OF ADJACENT TIPS NO MORE THAN TWO MAY HAVE THE SAME POLARITY AT ANY ONE TIME. 